Noise gates attenuate hum, hiss, and static in the signal by greatly diminishing the volume when the signal falls below a set threshold. Noise gates are often used by electric guitarist who play with vintage amps, which can have unwanted hum in the tone, and by guitarists from heavy metal who use high distortion levels, which add noise to the signal even when no notes are being played. Noise gates mute the signal when it falls below a certain threshold. This means that during bars of rest for the guitarist in a song, the hum or noise from the amp or distortion pedal will not be heard by the audience. Noise gates are expanders—meaning that, unlike compressors, they increase the dynamic range of an audio signal to make quiet sounds even quieter.[60] If used with extreme settings and combined with reverb, they can create unusual sounds, such as the gated drum effect used in 1980s pop songs, a style popularized by the Phil Collins song In the Air Tonight.
source: Wikipedia